A carbon fiber thread is generally produced by subjecting a carbon fiber thread precursor such as an acrylic fiber thread to an oxidization treatment at 200 to 300° C. under an oxidizing atmosphere to obtain an oxidized fiber thread followed by subjecting the oxidized fiber thread thus obtained to a carbonization treatment at 1,000° C. under an inert atmosphere. Such a carbon fiber thread has various excellent physical properties, so that it is widely used as a reinforcing fiber for various fiber reinforced composite materials, and a demand for it has been rapidly increased in recent years because it has been used in industry such as buildings, public works, and energy related fields besides the uses in airplanes and sport goods. Therefore, supply of the carbon fiber thread at lower cost is highly desired.
As a method for obtaining a carbon fiber thread at low cost, for example, a method is known in which a plurality of carbon fiber thread precursors wound up around bobbins or folded and piled up in boxes are continuously subjected to heat treatment (an oxidization treatment and a carbonization treatment) with an end of one precursor being connected to an end of another precursor. However, in this method, a jointed portion connecting respective ends of two carbon fiber thread precursors easily causes breakage of fibers by heat accumulation and so on while being subjected to heat treatment as compared with other portions. Therefore, the jointed portion is previously subjected to an oxidization treatment before heat treatment so as to prevent the breakage of fibers.
Specifically, a method is described in Patent Document 1, in which a back end of a preceding carbon fiber thread precursor and a front end of a succeeding carbon fiber thread precursor are connected by use of a fiber thread previously subjected to an oxidization treatment. In addition, a method is described in Patent Documents 2 and 3, in which two carbon fiber thread precursors each having at least one of a back end and a front end subjected to an oxidization treatment are connected. In addition, a method is described in Patent Document 4, in which, to detect a defect existing in a fiber thread bundle, a passing fiber thread is bent with a guide roller having a small radius of curvature and thereby the defect is caused to stick out from an outer circumference of the fiber thread bundle, and thus the portion stuck out is detected by an optical detection device.